The present invention relates to inhibiting corrosion that may result from post metal chemical-mechanical polishing (CMP) cleaning steps. More particularly, the present invention relates to inhibiting corrosion that may result from post metal chemical-mechanical polishing (CMP) cleaning steps by using a cleaning mixture including ethylene glycol and hydrofluoric acid.
A metal plug, typically a tungsten plug, is formed in a dielectric layer to provide a conductive pathway between a metallization layer and an underlying integrated circuit (IC) substrate layer and/or between two successive metallization layers disposed above the integrated circuit (IC) substrate. FIG. 1A shows a partially fabricated IC 10 that includes a tungsten layer 16, which is subjected to CMP to form tungsten plugs 24 as shown in FIG. 1B. CMP typically involves mounting a substrate face down on a holder and rotating the substrate face against a polishing pad mounted on a pallet, which in turn is rotating or is in orbital state. A slurry containing a chemical, e.g. an oxidizing agent such as Fe(NO.sub.3).sub.3, that chemically interacts with the tungsten layer and an abrasive, e.g. alumina (Al.sub.2 O.sub.3), that physically removes the tungsten layer, is flowed between the wafer and the polishing pad or on the pad near the substrate.
Referring to FIG. 1A, which shows partially fabricated IC 10 before the substrate surface undergoes CMP. A dielectric layer 12 disposed above a substrate surface 18 and having contact holes 14 is filled with tungsten. A tungsten layer 16 is also disposed atop dielectric layer 12. Some significant steps involved in forming the partially fabricated IC of FIG. 1A include blanket depositing dielectric layer 12, such as SiO.sub.2, on substrate surface 18. After dielectric layer 12 is planarized, a masking layer (not shown), which typically includes photoresist, is blanket deposited over dielectric layer 12 and patterned by conventional photolithography. Next, the unmasked portions of dielectric layer 12 are etched to form contact holes 14 that provide an opening to the underlying substrate layer. After the masking layer is removed, tungsten layer 16 is blanket deposited over the substrate surface, filling contact holes 14 with tungsten. Those skilled in the art will recognize that before the contact holes are filled with tungsten, they may be filled with a conductive titanium layer (not shown to simplify illustration) and a titanium nitride barrier layer (not shown to simplify illustration). Those skilled in the art will also recognize that vias may be similarly etched to provide an opening in a dielectric layer that is disposed between two metallization layers and that metal plugs may be formed in such vias similarly.
Next, tungsten layer 16 of partially fabricated IC 10 undergoes CMP, as described above, to form tungsten plugs by removing the tungsten layer deposited above the dielectric layer. Now referring to FIG. 1B, which shows a partially fabricated IC 20 after undergoing CMP. Partially fated IC has tungsten plugs 26 formed in a dielectric layer 12 disposed atop a substrate 18. During tungsten CMP, Fe(NO.sub.3).sub.3 oxidizes the tungsten to form tungsten oxide that is abraded by the alumina (Al.sub.2 O.sub.3) particles in the slurry. In this manner, the tungsten layer above the dielectric layer is removed to form tungsten plug 26 shown in FIG. 2.
During, CMP, the top portion of the dielectric layer may fracture when the substrate surface is subjected to pressures as high as 10 psi and/or may also develop microcracks or scratches due to the abrasive action of the slurry. Unfortunately, the iron residue from the slurry migrates into the fissures and microcracks formed at the top portion of the dielectric layer, i.e. between about 50 and about 75 .ANG.ngstroms at the top of dielectric layer 12, and forms a contaminated dielectric layer 30. The removal of iron contaminants from contaminated layer 30 is important because such contaminants lower the insulating properties of the dielectric layer, making the IC susceptible to catastrophic device failures.
In order to remedy this problem, a current approach includes treating the substrate surface after tungsten CMP with about 0.5% (by volume) hydrofluoric acid in deionized water to remove the iron contaminated dielectric layer. Unfortunately, prolonged exposure of the tungsten plug to a dilute hydrofluoric acid solution results in an increased likelihood of corrosion of the tungsten plug. Corrosion has been identified in those plugs where a characteristic "seam" or a "keyhole" type structure typically formed near the middle region of the plug undergoes enlargement.
What is therefore needed is an improved process which effectively inhibits metal plug corrosion resulting from post tungsten CMP cleaning steps.